Global protection from tropical cyclones by coastal ecosystems—past, present, and under climate change

Communities in low-lying coastal areas are increasingly at risk of extreme sea level events such as tropical cyclone (TC) storm surges, often exacerbated both by sea-level rise due to climate change and non-climatic drivers, including historical population and settlement trends (Oppenheimer et al 2019). With climate change, models show an increase in both the intensity and frequency of TCs (Knutson et al 2020, Emanuel 2021, Bloemendaal et al 2022). Therefore, climate change will likely exacerbate TC risk across the globe (Lin et al 2012, Collins et al 2019). The projected changes necessitate large-scale adaptation efforts globally, especially in coastal areas which are not only exposed to direct wind damage but also damage from resulting storm surges.

Coastal ecosystems such as mangroves, coral reefs, seagrass, coastal forests, and salt marshes have the potential to reduce disaster risk (Arkema et al 2013, Spalding et al 2014a, Beck et al 2018, Chaplin-Kramer et al 2019, 2023, Reguero et al 2019, 2021, Selig et al 2019, Menéndez et al 2020, Costanza et al 2021, Tiggeloven et al 2022). Benefits derived from nature are often encompassed in concepts such as nature-based solutions (NbS), ecosystem-based disaster risk reduction, and ecosystem-based adaptation (IUCN 2016, Sudmeier-Rieux et al 2021, UNDRR 2021, Cooley et al 2022). The mechanisms by which these ecosystems reduce TC associated risks include the reduction of wind speed, water retention, acting as a barrier to prevent flooding, and attenuating wave height and energy (Wamsley et al 2010, Shepard et al 2011, Duarte et al 2013, Pinsky et al 2013, Ferrario et al 2014, Guannel et al 2016, Narayan et al 2016, Sudmeier-Rieux et al 2021, Jordan and Fröhle 2022).

At the same time, coastal ecosystems are negatively affected by anthropogenic habitat degradation (Oppenheimer et al 2019). Thus, protection and restoration measures have been proposed to maintain the protective function of coastal ecosystems (Beck et al 2018, Worthington and Spalding 2018, Cooley et al 2022, Tiggeloven et al 2022). To maintain or even increase nature's role in coastal protection requires knowledge about the location of potential beneficiaries in relation to both coastal risk and coastal ecosystems. It is also necessary to identify historical and potential future changes in the patterns and magnitude of nature's protective potential (Spalding et al 2014b, Ruckelshaus et al 2020). Probabilistic risk assessment can provide a nuanced picture of risk since it reflects not only the severity of potential impacts but also the probability of a coastal risk materializing (Aznar-Siguan and Bresch 2019). While previous studies have estimated the number of beneficiaries of risk reduction through coastal natural habitat, there have been few studies that looked at changes in global ecosystem service provision, historically and with climate change (Arkema et al 2013, Beck et al 2018, Selig et al 2019, Menéndez et al 2020, Burke and Spalding 2022, Chaplin-Kramer et al 2023).

Using a combination of ecosystem index data and probabilistic risk modeling, we aim to answer the question: how has coastal protection by ecosystems changed over time and how will it evolve with a change in TC hazard due to climate change? We explore this question based on both historical and potential future changes of coastal ecosystems, population, and climate. Thereby, we quantify how many people on average are annually protected by coastal ecosystems currently and in the past taking into consideration population and ecosystem change. Looking to the future, we interrogate how coastal protection by ecosystems may change with climatic changes and large-scale efforts in nature restoration.

2.1. Data

2.2. Method

2.2.1. Impact and protection

This research uses the open-source probabilistic modeling platform CLIMADA to perform a TC risk assessment for populations in the vicinity of coastal ecosystems (Aznar-Siguan and Bresch 2019, Bresch and Aznar-Siguan 2021). Risk is defined as the probability of an event occurring multiplied by its severity and is obtained by combining the hazard (probabilistic TC set), exposure (population distributions), and vulnerability. Here we use a simplified vulnerability model and only count the number of people affected by every Saffir-Simpson TC scale value 1–5 to capture all potential ecosystem service beneficiaries (National Hurricane Center 2022). CLIMADA is spatially explicit, which means impact is measured by calculating which exposures (i.e. number of people) are located in an event's windfield. Hence, we obtain the severity, or impact, of each TC event in the probabilistic set as the number of people subject to a maximum wind speed of a given category. Subsequently, we compute the average annual impact, or risk, per exposure point by averaging over all events weighted by their annual occurrence probability. This thus covers yet differentiates regions that are regularly and rarely exposed to TCs.

Using a spatial overlay with InVEST coastal ecosystem data, we assess the absolute number and proportion of people simultaneously within the protection distance of coastal ecosystems, i.e. the radius within which coastal ecosystems have the potential to provide a protective service to coastal populations. The number of people protected by coastal ecosystems then corresponds to the sum of all impacted people that are within the protection distance of coastal ecosystems (see section 2.1.2). Thus, protection occurs if there is a non-zero probability of a TC at the given location, and population exposure within the protective distance of coastal ecosystems. Conversely, non-protection occurs if a population exposure is impacted outside the protective distance of any coastal ecosystem. The average number of people impacted annually can be aggregated across exposure points, for example to calculate the average number of people impacted in a certain region, or a given coastal ecosystem category, per year. The proportion of people protected (in %) refers to the fraction of people impacted within the protection distance of coastal ecosystems. Note, this is a proportion of the total number of people impacted (i.e., experience TCs and are at an elevation below 10 m above sea level), not a share of the total number of people in the exposure dataset.

2.2.2. Comparison across scenarios

We explore how the protection of people in the low-elevation coastal zone by coastal ecosystems evolves over time based on changes in the population distribution, the coastal ecosystems themselves, and the TC climatology. A baseline is established to reflect the current level of coastal protection through ecosystems based on ecosystem service and population data for 2020, as well as TC hazard data from the STORM model based on the current climatic conditions. We then quantify changes from the baseline: first, historical changes due to developments in population and ecosystems between 1992 and 2020 are investigated. Next, potential changes in impact and protection due to a changed hazard under climate change until 2050 are analyzed. Finally, the impact of mangrove restoration is assessed. Figure 1(a) shows protection in 1992 (historical), 2020 (baseline) and 2050 (future under climate change), while figure 1(b) shows how changes in the population distribution, the coastal ecosystems themselves, and the TC climatology affects the relative share of protected people. Below, we first discuss the geographical patterns of protection in the baseline (section 3.1), before further investigating historical and future changes, and the factors influencing these changes (sections 3.2–3.4).

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3.1. Current baseline

On average every year 14 million people benefit from protection from TCs by coastal ecosystems worldwide (see middle bar in figure 1(a)). This equates to one in five (21%) of all people in the low elevation coastal zone currently impacted by TC being within the protection distance of coastal ecosystems (see middle bar in figure 1(b)). To contextualize this annual average over a time period relevant for disaster risk management and adaptation planning, this means that over a time period of 50 years, the expected number of people protected globally would be 700 million. Of those protected, the highest numbers of people are seen for high levels of protection by coastal ecosystems and low wind speed categories (figure 1(a), supplementary material table 4).

Global scale absolute and relative numbers of people protected by coastal ecosystems only provide limited information, since the patterns and levels of protection are not evenly distributed. The highest number of people impacted are found in Eastern- and South-Eastern Asian countries (figure 2(a), table 2), due to a combination of high population density and exposure to high levels of TC hazards. Very high and high degrees of protection, as well as a complete lack of protection at individual population exposure points, can be found across global coastlines (figure 2(b)). However, the share of impacted population within coastal ecosystem protection distance differs considerably between geographical regions. For instance, the difference in the proportion of people protected between Southern Asia and the Caribbean, where the lowest and highest share can be found (8% and 53% respectively), is 45% (figure 3(a)). The highest absolute number of people protected from TC by coastal ecosystems are found in the Philippines, China, and Japan (table 2). However, the relative proportion of people protected is highest for countries in the Caribbean and Pacific Islands, where 69%–92% of the impacted people are within the protection distance of coastal ecosystems (table 2).

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Table 2. Highest ranking countries or areas in terms of the absolute number of people impacted annually, highest number of people protected, and the proportion of people protected. For each high-ranking country in one of these categories, the rank across the other categories is shown as well. The column 'Total number of people impacted annually' refers to the average number of people impacted by tropical cyclones per country each year. 'Number of people protected annually' gives the average number of people protected per country each year, while 'Proportion of people protected' shows the people protected as a share of people impacted per country.

Highest ranking countries for absolute and relative protection
Total number of people impacted by tropical cyclones annually			Number of people protected annually			Proportion of people protected (%)
1	China	30 M	1	Philippines	5 M	1	US Virgin Islands	92%
2	Japan	11 M	2	China	4 M	2	Saint Vincent and the Grenadines	84%
3	Philippines	9 M	3	Japan	2 M	3	Saint Kitts and Nevis	72%
4	Taiwan ROC	3 M	4	Hong Kong SAR	0.8 M	4	Hong Kong SAR	70%
5	Viet Nam	2 M	5	South Korea	0.5 M	5	Northern Mariana Islands	69%
...
9	South Korea	1 M	6	Taiwan ROC	0.5 M	21	Philippines	51%
10	Hong Kong SAR	1 M	8	Viet Nam	0.3 M	38	South Korea	35%
53	Northern Mariana Islands	17 K	38	Saint Vincent and the Grenadines	13 K	52	Japan	15%
55	Saint Vincent and the Grenadines	16 K	40	US Virgin Islands	13 K	53	Taiwan ROC	15%
56	US Virgin Islands	14 K	42	Northern Mariana Islands	12 K	56	China	12%
59	Saint Kitts and Nevis	12 K	51	Saint Kitts and Nevis	8 K	57	Viet Nam	11%

3.2. Past changes: population development and coastal ecosystem loss

3.3. Potential effect of climate change

3.4. Potential changes under nature protection and restoration scenarios

Under the mangrove restoration scenario and considering changes in TC due to climate change, every year 109 000 more people may be protected from TCs by coastal ecosystems respectively. The fraction of people protected would thereby increase by 0.1% (figure 4). It is noticeable that the increase in the degree of protection by coastal ecosystems is higher than switches from non-protection to protection (209 000 people annually, which corresponds to a relative protection increase of 0.2%, or 6.27 million over 30 years) (figure 4). This results from restoration occurring mostly in areas which already provide some level of protection rather than areas which are completely unprotected.

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The global increase in protection is relatively minor compared to historical losses; however, mangroves are confined to low and mid latitudes. On a country-scale, much higher gains in the level of coastal ecosystem-based protection are discernable than globally. Based on the global observation that changes occur mostly in terms of shifts within levels of ecosystem-based protection, we look at increases in the fraction of people under very high protection, rather than changes from non-protection to protection (table 3). The highest country-level increase occurs for mangrove restoration in Bermuda (39% increase in the fraction under very high protection), although increases for other countries are in the low single digits (table 3). For countries with currently small mangrove extents, even minor increases in mangrove land cover can result in comparatively high increases in protection. Smaller countries and island states benefit especially.

Table 3. Country-level changes in very high protection under the mangrove restoration scenario with climate change (SSP585) in 2050. The five countries with the biggest changes are listed, both in terms of absolute and relative changes.

Countries with biggest changes in very high protection under mangrove restoration scenario
Country	Number of people impacted annually	Baseline value under very high protection	NbS scenario value under very high protection	Difference between baseline and scenario
Absolute change
China	45.9 M	1.4 M	1.5 M	103.4 K
Philippines	11.0 M	2.1 M	2.2 M	35.9 K
Bermuda	8.2 K	3.5 K	6.6 K	3.2 K
Trinidad and Tobago	42.1 K	14.4 K	15.9 K	1.5 K
Barbados	30.1 K	9.8 K	11.3 K	1.5 K
Relative change
Bermuda	8.2 K	42.6%	81.3%	38.7%
Barbados	30.1 K	32.5%	37.4%	4.9%
Trinidad and Tobago	42.1 K	34.2%	37.7%	3.5%
Papua New Guinea	6.3 K	31.1%	32.3%	1.2%
Sri Lanka	30.3 K	6.7%	7.5%	0.8%

Globally, a minority (21%) of all people impacted by TCs in coastal areas are currently within the protection distance of coastal ecosystems. However, in certain countries, a large majority of impacted people are protected, which highlights the importance of coastal ecosystems, as well as the need for analyzing NbS across a variety of spatial scales. In terms of absolute numbers of people, highly populated countries in Eastern and South-Eastern Asia are the chief beneficiaries of the coastal protection from TCs of mangroves and coastal forests, coral reefs, sea grass, salt marshes and wetlands. However, in proportional terms, protection by coastal ecosystems is key for small island states and developing countries, especially in the Caribbean and Pacific Islands.

In this study, we use maximum sustained wind speed as a proxy for all TC sub-hazards (storm surges, heavy precipitations, floods, landslides, etc), and therefore cannot model the physical mechanisms (e.g. wave attenuation or water retention) through which coastal ecosystems reduce risk. Thus, the risk reduction reported here relies on the assumption that sub-hazard intensities (e.g. precipitation and surge wave heights) and the derived impacts are correlated to the maximum wind speed, which may not always be the case. This relationship should be treated with caution at local scales but is reasonable over large spatial extents and compatible with the resolution and sophistication of the ecosystem model. Hence, we modeled the ecosystem service in a semi-quantitative fashion only: the ecosystem risk reduction potential is a ranking score, and the hazard intensity is categorized into the Saffir-Simpson scale. Protection is provided as soon as a pixel is below 10 m elevation, is within 2 km of an ecosystem, and is hit by at least one TC of the probabilistic cyclone set. Our research therefore only focuses on areas affected by TCs, but coastal ecosystems may provide important coastal protection benefits in other areas as well. Since this study focuses on coastal protection as an ecosystem service, we did not consider grey or hard infrastructure options. However, it is important to bear in mind that unprotected people by our definition may still receive protection by this infrastructure, and that areas with low restoration potential may benefit from other protective measures.

Furthermore, both absolute and relative numbers of people protected are sensitive to choices e.g. with regards to the protective distance chosen (see supplementary material tables 1–3). While some studies have modeled hazard attenuation through coastal ecosystems for both cyclonic and non-cyclonic flooding more explicitly, they tend to focus more on economic damages averted, and to a lesser extent people protected (Beck et al 2018, Menéndez et al 2020, Tiggeloven et al 2022). Simultaneously, studies from a more human-centric or ecosystem service provision perspective frequently use non-probabilistic hazard indices (Arkema et al 2013, Chaplin-Kramer et al 2019, Selig et al 2019). Our research combines the strengths of both approaches by providing a probabilistic risk assessment for populations in proximity to coastal ecosystems. This is well suited for identifying spatial and temporal patterns in the protective function of coastal ecosystems at scale.

The historical decrease of relative protection by coastal ecosystems over the past 30 years caused by coastal ecosystem loss (figures 1(b) and 3) is concerning. Other studies have demonstrated the negative consequences of ecosystem loss for coastal protection, both for moderate changes in coastal vegetation and complete loss scenarios (Arkema et al 2013, Beck et al 2018, Menéndez et al 2020, Tiggeloven et al 2022). Our findings complement these perspectives by (i) identifying where the decrease in protection has occurred historically, and (ii) quantifying the magnitude of changes, as well as the relative effects of population and ecosystem changes. Previous research has shown that coastal areas are subject to both increases in population and ecosystem degradation, and that both factors play a role in increasing coastal disaster risk (Oppenheimer et al 2019, Cooley et al 2022). We further find that ecosystem loss is the main factor, but there are strong regional variations across the globe.

The increase in the absolute number of people protected, due to an exacerbated hazard under climate change, which results in a higher number of people impacted, can give the impression that protection by coastal ecosystems is increasing (figure 1(a)). However, the projected TCs will predominantly affect areas which are currently unprotected by coastal ecosystems. Hence, the proportion of people protected by coastal ecosystems is decreasing, even though the overall increase in TC frequency leads to more people being protected. It is concerning that a decrease in relative protection is observed due to changes in TC activity alone, since extrapolating historical coastal ecosystem losses as well as population growth would lead to an even more pronounced reduction in coastal protection through ecosystems. In addition, the STORM model only considers climate change in the intensity and frequency of TCs. Hence, coastal risk aggravating factors such as sea-level rise or erosion are not included. These, along with potential adverse effects of higher storm frequencies on the coastal ecosystems, are likely to further increase the risk and thus the need for coastal ecosystem protection (Beck et al 2018, Schuerch et al 2018, Cooley et al 2022).

As a possible adaptation measure, we considered the potential for mangrove restoration (Narayan et al 2016, Worthington and Spalding 2018, Menéndez et al 2020, Rana et al 2022). However, at the global scale changes from non-protection to protection are fairly small. Thus, NbS should not be seen as a silver bullet for disaster risk and climate adaptation. Yet, the potential for offsetting some of the decreases in protection due to historical habitat loss should not be underestimated. Not all areas are equally suitable for mangrove restoration, with the most urbanized areas being the least suited both from an ecological and from a socio-economic point of view (Worthington and Spalding 2018). Areas where coastal ecosystems already protect a high share of the coastal population are of interest (e.g. in small island states and developing countries), as well as areas where nature's protective value has been degraded in the recent past. Note that we specifically considered restoration which implies that areas that never had ecosystem protection cannot be targeted. Our scenario focuses on mangroves as they give the highest level of protection, provide a wide range of other co-benefits, and restoration is already ongoing in a number of countries (Spalding et al 2014b, Worthington and Spalding 2018, Das et al 2022, Gerona-Daga and Salmo 2022). This does not mean that restoration efforts for other coastal ecosystems such as seagrass, saltmarsh or wetlands are not of value. Indeed, it suffices to look at the historical decrease in protection due to ecosystem losses to realize their potential if restored. Overall, our analysis of both the past and future changes in protection underpin the necessity of global ecosystem preservation and restoration efforts for meeting adaptation goals in a changing climate also highlighted in previous studies (Spalding et al 2014b, Beck et al 2018, Tiggeloven et al 2022). Simultaneously, we must caution that the effects of the TCs and the changing climate on the coastal ecosystems themselves, which were not considered here, might jeopardize the NbS adaptation efforts. This may lead to an overestimation of the areas considered to be restorable under climate change. Future research can attempt to model these multi-directional interactions of coastal risk, climate change, and coastal ecosystems.

Through the integration of index-based ecosystem data and probabilistic risk modeling, we identify patterns of coastal protection in relation to cyclone risk at different spatial and temporal scales. Our findings show how ecosystem-based coastal protection has reduced over the last 30 years due to ecosystem and population change, as well how it may change over the next 30 years based on climatic changes and potential nature protection and restoration activities. Our results provide insights on the contribution NbS can make to climate change adaptation globally, and how benefits are distributed across different countries. Currently, one in five people (21%) impacted by TCs in the low elevation coastal zone experience some level of protection from coastal ecosystems, such as coral reefs, mangroves, coastal forests, seagrass, salt marshes and wetlands. This share has decreased by 2% over the last 30 years mainly due to coastal ecosystem degradation and would be even more distinct without the simultaneous increase of coastal population in protected areas. With climate change and current population, 40% more people will be impacted by TCs annually, yet the share of annually protected people decreases by 1%, which amounts to 31.5 million over 30 years. The potential to increase coastal protection by restoring nature is not distributed evenly across the globe, but very high in smaller countries and island states. Our analysis can support prioritization of areas for more localized assessments of the applicability of NbS for disaster risk reduction and climate change adaptation.

The authors are grateful to Simona Meiler and Nadia Bloemendaal for discussing options regarding the analysis of the TC data. We would like to thank Mark Spalding and two anonymous reviewers for their insightful comments on this article. This research has received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 101003687 and was supported by a grant from the Bezos Earth Fund to The Nature Conservancy for research into nature-based climate mitigation and adaptation.

The data that support the findings of this study are openly available at the following DOI: 10.3929/ethz-b-000626330.

Code is available under

https://github.com/CLIMADA-project/climada_papers/tree/main/202305_coastal_ecosystems_TC (Hülsen 2023a, 2023b).